Marginal Thermobaric Stability in the Weddell Sea

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Marginal Thermobaric Stability in the Weddell Sea

• Miles McPhee
• McPhee Research Company

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Thermobaric Instability Following Loyning and
Weber, JGR, 102, p. 27875
2-layer system, upper layer colder and less saline
Linearized equation of state
Thermal expansion coefficient increases with depth
ambient
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Weddell Sea
Greenland Sea
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Strength of thermobaric tendency must exceed the
background stratification
Marginal stability line
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ANZFLUX Ship CTD station 50, linearized
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2.4 hour average of turbulence measurements
centered at time 206.35 (Warm Regime drift).
Circles are averages lines are twice the std dev
of the 15-min samples.
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Two-layer (Type II) stability diagram following
Akitomo (1999) for idealized Ship Station 50
Increase Sml
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• The thermobaric barrier calculation
• Calculate the actual density (pressure included),
  subtract density of a water column with mixed
  layer properties. Determine the level (zmax) of
  the maximum difference drmax.
• Determine the sensible heat that must be vented
  to reduce water temperature above zmax to ?ml.
• Add the latent heat loss required to increase
  salinity (by freezing) enough to eliminate ?? at
  zml
• Htot is the total heat loss.

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Pentagrams indicate Hto t lt 100 MJ/m2
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27.4 W m-2
McPhee, Kottmeier and Morison, JPO, 1999
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Ice temperatures from the AWI Buoy thermistor
string
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Mean values almost identical 30 W m-2
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Friction velocity prescribed from buoy results.
Heat flux also from buoy ice measurements but
scaled by the calculated ice thickness
Ocean heat flux calculated prognostically, ice
thickness determined by enthalpy balance at the
interface.
Dynamic mixed layer depth based on buoyancy
frequency (pot density). Scalar based on
difference from near surface value.
The model neglects thermobaric effects but
calculates thermobaric barrier parameters at each
time step.
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The 1-D model forced with buoy data and
initialized with YU075 No thermobaricity effect
considered.
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The 1-D model forced with buoy data and
initialized with YU075 Eddy viscosity set to 2000
cm2/s across vertical domain after 217.75.
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Horizontally Homogeneous Model Results

• Initialize model with every profile with Htot lt
  100 MJ m-2 forced by buoy time series (38)
• 27 1-D profiles became unstable by the end of
  August

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Is there a simple way of getting a handle on eddy
viscosity and scalar diffusivity when
thermobaric mixing is occurring?

1. Parameterize entrainment process in terms of
   conversion of PE to TKE
2. Base the mixing length on a fraction (k) of the
   entrained layer depth
3. Then

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Summary

• Thermobaric instability was not observed directly
  during the ANZFLUX 94 project but there is a
  strong inference that it occurred shortly after.
• About ¼ of the profiles observed with the yoyo
  CTD system during the Maud Rise drift went
  thermobarically unstable by the end of winter in
  a simple 1-D model forced with drifting buoy
  data.
• In the model, preconditioning of the initial
  density profile to include distinct step-like
  structure in the upper pyncnocline was necessary
  for instability.
• Steps were found mostly in the halo region
  surrounding Maud Rise (2500-3000 m isobaths)

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Summary (cont)

• There may be a good chance of encountering
  episodes of Type II convection near Maud Rise in
  late winter.
• Measuring turbulent dissipation rates and
  turbulent fluxes directly during a Type II
  episode is feasible based on ANZFLUX experience.
  Such data would be of great value in evaluating
  and guiding numerical model development.
• Even in the absence of direct Type II convection,
  studying processes that maintain the step
  structure and pycnocline weather in the Weddell
  would add significantly to our understanding of
  the system.